The use of redispersible film forming polymers in powder form finds widespread use throughout a number of industries. For example, such redispersible film forming polymers are used as construction binders for tile adhesives, synthetic resin plasters, floor levelling mixes, EIFS (exterior insulating finishing systems) and in areas where a waterless system is generally desirable. These systems in general are produced by the dry blending of the raw materials, such as sand, cement, calcium carbonate, silica flour, modified cellulose based thickeners, and the dry polymer powder, to obtain a ready to use finished product for site mixing where water is added at the point of use. Such systems are advantageous as they avoid the need for two separate prepackaged components, one component containing an aqueous polymer dispersion and the other component containing the powder component which may contain a hydraulic setting component such as cement (i.e. so-called “two component systems). Avoiding such systems may be advantageous for a number of reasons such as economy, utility and environmental considerations.
To obtain a film forming polymer in powder form, a liquid dispersion of the polymer is subjected to a drying operation where the water is removed. The method may include, for example, spray drying or freeze drying. Spray drying is a widely used and understood method that gives a fine powder which usually does not require further processing. This method is therefore generally preferred in the industry.
In order to manufacture a redispersible film forming polymer powder with glass transitions below 50° C., it is generally necessary to add to the liquid dispersion before spray drying or during polymerisation of the dispersion polymer a quantity of redispersing aid. Redispersing aids act to coat the individual polymer particles to prevent irreversible primary particle formation during drying operations of the polymer dispersion. Such aids also advantageously increase the yield during the spray drying process by minimizing sticking of the dried polymer to the walls of the spray dryer. The redispersing aid may also improve the storage stability of the polymer powder by minimizing “blocking” (the thermoplastic fusion of the polymer particles under weight pressure), and should generally have some hydrophilic properties to aid redispersability of the polymer powder on the reintroduction of water.
The redispersible polymer powders known to date generally comprise a redispersing aid that is water soluble and which is generally added to the polymer dispersion before spray drying. As stated above, this advantageously prevents or reduces the formation of primary particles during the spray drying operation. Alternatively, the redispersing aid may be added during polymerisation of the liquid polymer dispersion to aid in colloidally stabilising the polymer during polymerisation and as a redispersing aid during spray drying.
The use of salts of arylsulfonic acid—formaldehyde condensates as redispersing aids is known, especially for acrylic dispersions. German Offenlegungsschrift 24 45 813 translated as “Redispersible synthetic powder and method of production” describes the use of phenol sulfonic acid formaldehyde condensates and naphthalene sulfonic acid formaldehyde condensates and their alkali salts or their alkali earth metal salts as redispersing aids for redispersible powders. U.S. Pat. No. 5,225,478 assigned to BASF also describes the use of phenol sulfonic acid formaldehyde condensates and their alkali salts or alkali earth metal salts as a redispersing aid. U.S. Pat. No. 6,028,167 assigned to BASF describes an improvement by controlling the molecular weight of the redispersing aids. Likewise, Australian Patent No. 718,907 describes an improvement in naphthalene sulfonic acid formaldehyde condensates by controlling or restricting the molecular weight of the redispersing aids.
Australian application number 2003904725 to Acquos Pty Ltd describes an ortho-cresol sulfonate condensate based oligomer where a low molecular weight product is obtained with high molecular weight condensates practically eliminated. This provides for considerable improvements in the properties of the manufactured redispersible powders. WIPO application number WO2006/034531 provides for further improvements.
Whilst the abovementioned low molecular weight product aryl sulfonic acid formaldehyde condensates, such as the phenol sulfonic acid formaldehyde condensate as in U.S. Pat. No. 6,028,167 and the naphthalene sulfonic acid formaldehyde condensate in Australian Patent No. 718,907, are effective redispersing aids, with Australian application number 2003904725 being an especially effective redispersing aid, they suffer from some minor drawbacks such as colour and set time retardation at large polymer dosage levels.
Polyvinyl alcohols have historically been used as redispersing aids for ethylene vinyl acetate (EVA) dispersions for many years. For example, U.S. Pat. No. 3,883,489 assigned to Hoechst Aktinengellschaft makes use of polyvinyl alcohol as a redispersing aid for ethylene vinyl acetate dispersions. Whilst not as effective, polyvinyl alcohols can also be used as redispersing aids in the manufacture of redispersible acrylic or styrene-acrylic polymer powders. For example, U.S. Pat. No. 5,567,750 assigned to Wacker-Chemie GmbH makes use of a polyvinyl alcohol along with an amino functional polyvinyl alcohol for the manufacture of a redispersible styrene-acrylic polymer powder. U.S. Pat. No. 5,519,084 assigned to Air Products and Chemicals makes use of a polyvinyl alcohol as a redispersing aid with an acrylic polymer consisting of up to 15% olefinically unsaturated carboxylic acid.
The use of polyvinylalcohol as a protective colloid in the preparation of vinyl acetate-acrylate (or vinyl acrylic) ester copolymer (optionally with ethylene) is mentioned in U.S. Pat. No. 6,706,805. Although these polymers are effectively redispersible, the polymer is formed in two stages. The first stage involves polymerisation of the vinyl ester component to a conversion of from 90-100% by weight and the second stage the addition of an ester of (meth)acrylic acid and subsequent polymerisation. Whilst it would be desirable to have an even distribution of alkyl (meth)acrylate along the vinyl acetate based backbone to render the polymer more alkali resistant and hydrophobic, none of the current technologies based exclusively upon polyvinyl alcohol aids are entirely satisfactory for redispersible polymer powders.
The redispersible polymer powders commercially offered to date may hence be classified into systems that are:                1) Polyvinyl alcohol based;        2) Alkyl aryl sulfonate based;        3) Proprietary colloid based; or        4) Core shell technology based.        
The most popular commercial redispersible polymer powders are based on either polyvinyl alcohol or alkyl aryl sulfonate technology. To date, no mention or successful attempt has been made involving the use of a polyacrylamide protective colloid as the main protective colloid, or one of the main protective colloids, during formation of a redispersible polymer powder, for example based on vinyl acrylic technology.